In the eukaryotic cell cycle, several positive and negative factors control the cell cycle progression. Among the positive factors, the protein kinase family plays an important role. Each member of the family comprises a regulatory subunit, or cyclin, and a catalytic subunit named cyclin-dependent kinase (cdk). A number of reports have suggested that cyclin D-cdk4, cyclin D-cdk6, and cyclin E-cdk2 play important roles in promoting the transition from the G1 phase to the S phase by the phosphorylation of retinoblastoma protein (pRB). Recently, one further level of control has become apparent, namely the expression of cdk inhibitors (Sherr, C. J. and Roberts, J. M. (1995) Genes & Dev. 9:1149-1163). Two families of cdk inhibitor with different modes of action have already been identified in mammalian cells. One group, comprised of related proteins known as p21.sup.Cip1, p27.sup.Kip1, and p57.sup.Kip2, appears to function as specific inhibitors of the cyclin/cdk complexes (Harper, J. W., Adami, G. R., Wei, N., Keyomarsi, K. and Elledge, S. J. (1993) Cell 75: 805-816; Polyak, K., Lee, M.-H., Erdjument-Bromage, H., Koff, A., Roberts, J. M., Tempst, P., and Massague, J. (1994) Cell 78: 59-66; Toyoshima, H. and Hunter, T. (1994) Cell 78: 67-74; Matsuoka, S., Edwards, M. C., Bai, C., Parker, S., Zhang, P., Baldini, A., Harper, J. W., and Elledge, S. J. (1995) Genes & Dev. 9: 650-662). The second family of the cdk inhibitors is called INK4 family proteins. The four members of this family, called p15, p16, p18, and p19, bind directly to cdk4 and cdk6, and are therefore specific inhibitors of the cyclin D-dependent kinases (Hannon, G. J. and Beach, D. (1994) Nature 371: 257-261; Serrano, M., Hannon, G. J., and Beach, D. (1993) Nature 366: 704-707, Hirai, H., Roussel, M. F., Kato, J., Ashmun, R. A., and Sherr, C. J. (1995) Mol. Cell. Biol. 15: 2672-2681).
Although the precise roles of p27.sup.Kip1 are far from clear, its level decreases when cells are stimulated to enter the cell cycle, and increases when cells are arrested by either the change in TGF-.beta. concentration or contact inhibition (Polyak, K., Kato, J., Solomon, M. J., Sherr, C. J., Massague, J., Roberts, J. M., and Koff, A. (1994) Genes & Dev. 8: 9-22). p27.sup.Kip1 was cloned as a binding protein with cyclin E-cdk2 (Polyak, K., Lee, M.-H., Erdjument-Bromage, H., Koff, A., Roberts, J. M., Tempst, P., and Massague, J. (1994) Cell 78: 59-66) or cyclin D-cdk4 (Toyoshima, H. and Hunter, T. (1994) Cell 78: 67-74). p27.sup.Kip1 inhibits the activity of most cyclin-cdk complexes and can inhibit the phosphorylation of cyclin-cdk complexes by CAK (cdk-activation kinases) (Kato. J., Matsuoka, M., Polyak, K., Massague, J., and Sherr, C. J. (1994) Cell 79: 487-496). Therefore, p27.sup.Kip1 functions as a negative regulator of the G1/S progression.
Tumor-specific mutations of the p27.sup.Kip1 gene are rare, whereas several cell cycle regulators, such as p16, p53, and pRB, are frequently mutated in some cancers and have been shown to be tumor suppressor genes (Ponce-Castaneda, M. V., Lee, M.-H., Latres, E., Polyak, K., Lacombe, L., Montgomery, K., Mathew, S., Krauter, K., Sheinfeld, J., Massague, J., and Cordon-Cardo, C. (1995) Cancer Res. 55: 1211-1214). However, the p27.sup.Kip1 -deficient mice were observed to have increased body size, multiple organ hyperplasia, retinal dysplasia, and formation of pituitary tumors (Fero, M. L., Rivkin, M., Tasch, M., Porter, P., Carow, C. E., Firpo, E., Polyak, K., Tsai, L.-H., Broudy, V., Perlmutter, R. M., Kaushansky, K., and Roberts, J. M. (1996) Cell 85: 733-744; Kiyokawa, H., Kineman, R. D., Manova-Todorova, K. O., Soares, V. C., Hoffman, E. S., Ono, M., Khanam, D., Hayday, A. C., Frohman, L. A., and Koff, A. (1996) Cell 85: 721-732; Nakayama, K., Ishida, N., Shirane, M., Inomata, A., Inoue, T., Shishido, N., Horii, I., Loh, D. Y., and Nakayama, K. (1996) Cell 85: 707-720). These data are in part similar to the case of the RB heterozygous knockout mice (Hu, N., Gutsmann, A., Herbert, D. C., Bradley, A., Lee, W.-H., and Lee, E. Y.-H. P. (1994) Oncogene 9:1021-1027). In addition, it has been shown that lower expression of the p27.sup.Kip1 protein correlated with poorer survival in breast cancer and colorectal cancer (Porter, P. L., Malone, K. E., Heagerty, P. J., Alexander, G. M., Gatti, L. A., Firpo, E. J., Daling, J. R., and Roberts, J. M. (1997) Nature Medicine 3: 222-225; Catzavelos, C., Bhattacharya, N., Ung, Y. C., Wilson, J. A., Roncari, L., Sandhu, C., Shaw, P., Yeger, H., Morava-Protzner, I., Kapsuta, L., Franssen, E., Pritchard, K. I., and Slingerland, J. M. (1997) Nature Medicine3: 227-230; Loda, M., Cukor, B., Tam, S. W., Lavin, P., Fiorentino, M., Draetta, G. F., Jessup, J. M., and Pagano, M. (1997) Nature Medicine 3: 231-234). These results clearly indicate that p27.sup.Kip1 plays an important role in inhibiting tumor formation and tumor progression. There have also been reports on the importance of the p27.sup.Kip1 gene in enhancing the susceptibility of tumor cells to anticancer drugs and in influencing the prognosis factors of cancers (Croix, B. S., Florenes, V. A., Rak, J. W., Flanagan, M., Bhattacharya, N., Slingerland, J. M., and Kerbel, R. S. (1996) Nature Medicine 2: 1204-1210; Loda, M., Cukor, B., Tam, S. W., Lavin, P., Fiorentino, M., Draetta, G. F., Jessup, J. M., and Pagano, M. (1997) Nature Medicine 3: 231-234; Hengst, L. and Reed, S. I. (1996) Science 271: 1861-1864; Pagano, M., Tam, S. W., Theodoras, A. M., Beer-Romero, P., Sal, G. D., Chau, V., Yew, P. R., Draetta, G. F., and Rolfe, M. (1995) Science 269: 682-685). Consequently, it has been desired to develop drugs that regulate the transcription of the p27.sup.Kip1 gene in order to prevent or treat malignant tumors.
Recent reports showed that p27.sup.Kip1 mRNA is induced by vitamin D3 in U937 cells (Liu, M., Lee, M.-H., Cohen, M., Bommakanti, M., and Freedman, L. P. (1996) Genes & Dev. 10: 142-153) and by neuronal differentiation (Poluha, W., Poluha, D. K., Chang, B., Crosbie, N. E., Schonhoff, C. M., Kilpatrick, D. L., and Ross, A. H. (1996) Mol. Cell. Biol. 16: 1335-1341). These facts suggest that the transcriptional regulation of the p27.sup.Kip1 gene might also be important in cellular differentiation.